I made a chicken dish with a mustard sauce and it's just too sharp. How can I counteract that taste?

If you don't want to add sweetness, say with honey, you could use white wine or sherry. If you want to stay more authentic to the recipe or not add calories and fat, you could use chicken broth. Good luck!

two ways to counteract sharpness are to add sweet or rich (fatty) ingredients. Depending on what else is in the recipe and what your eaters prefer, some suggestions are jam or dried fruits (as in Italian mostarda sauce), maple syrup, pomegranate molasses, avocado as a garnish to mix in with the sauce.

A couple of options: add sweetener, cream/butter, or both.